a. Field of the Invention
The present invention relates to a punch for use in a tool for clinch joining ductile materials, such as metal sheets, and in particular to a punch for use with a lanced clinch joiner or a round clinch joiner.
b. Related Art
It is known to join a plurality of sheets of ductile material by causing these to be deformed into an interlocking configuration in a local area. Such joins are made by ductile material joining tools comprising a die with an aperture that is opposite a punch assembly comprising a punch. The punch has a punch tip, which may have a flat or rounded punch surface. The edges of the punch surface may be radiused, chamfered, or sharp. The punch itself may be surrounded by a stripper mechanism to facilitate removal of the punch from the deformed ductile material. Layers of ductile material are sandwiched between the punch assembly and when the punch tip punch is pressed towards the aperture, material is drawn into the aperture. The material undergoes plastic deformation in the aperture to flow into a shape in which two or more layers are interlocked, for example by the forming of one layer around another layer.
The die aperture normally has a base with an anvil having an anvil surface and at least two side walls formed from movable blades. The blades are generally transverse to the anvil surface and extend in the direction in which the die and punch are pressed together. The blades help define the local area, for example a circular, square or rectangular area, in which the deformation of the layers of sheet material takes place. Once the material has been drawn and flows into the aperture, the blades move away from each other in a radial direction as the sheet material is compressed it flows laterally. Some types of die blade pivot outwards about a pivot mechanism below the level of the anvil surface. The pivot mechanism has a pivot axis or pivot point below and laterally outside an edge of the anvil surface.
A circular die and punch can be used to form a clinch joint in which sheet material is symmetrically deformed both axially and radially to form a leak-proof button, for example as disclosed in patent documents U.S. Pat. No. 5,150,513 and EP 1 055 467 A2. A square or rectangular die and punch can be used to form a trapezoidal clinch joint (also called a lance joint), in which the sheet material is cut through by the punch along a pair of parallel opposed lines, with the layers of sheet material deformed laterally outwards underneath each of the cuts, as disclosed in patent document GB 2,334,474. The present invention relates a punch for forming these and other types of joints in layers of ductile material.
The lifetime of a punch assembly is limited essentially by wear of the punch tip, either of the punch surface itself or edges to the punch tip. Such wear can be minimised if the punch tip is made from hardened steel (62 to 64 RC), but this increases the possibility of the punch tip breaking if the punch is not properly aligned with the die. The clearance between the punch tip and die blades for a lanced joint is normally very close, for example of the order of 50 μm for a 3 mm width punch tip used to join two pieces of 0.7 mm thick metal. The punch tip may therefore be damaged in use if it is not properly aligned with the die.
The corner of the die blade facing inwards to the die recess normally forms a sharp edge of about 90°, but this will become dull with excessive use. When clinch joining hard metals, such as stainless steel, the lifetime of a die blade may be as short as 10,000 to 20,000 cycles. In order to maintain join quality, it is necessary to keep to a conservative schedule for changing die blades, which adds to manufacturing cost.
When a punch tip becomes worn or damaged, the punch can be removed from the punch assembly and replaced with a new punch. However, because the punch is made from precision machined and hardened steel, each punch is relatively expensive. In addition, a stock of new punch tips may have to be maintained in order to avoid disruption in a production environment. The need to maintain a stock of punches adds to manufacturing cost, particularly if more than one type of punch is to be used with a particular punch assembly.
It is an object of the present invention to provide a more convenient punch for use in a tool for joining ductile materials.